This invention relates generally to apparatus for mixing at least two substances, such as dry cement and water. This invention relates more particularly, but not by way of limitation, to an inlet flow mixer and apparatus incorporating an inlet flow mixer with which a cement slurry can be formed for use in an oil or gas well.
After the bore of an oil or gas well has been drilled, typically a tubular string, referred to as casing, is lowered and secured in the bore to prevent the bore from collapsing and to allow one or more individual zones in the geological formation or formations penetrated by the bore to be perforated so that oil or gas from only such zone or zones flows to the mouth of the well. Such casing is typically secured in the well bore by cement which is mixed at the surface, pumped down the open center of the casing string and back up the annulus which exists between the outer diameter of the casing and the inner diameter of the well bore.
The mixture of cement to be used at a particular well usually needs to have particular characteristics which make the mixture, referred to as a slurry, suitable for the downhole environment where it is to be used. For example, from one well to another, there can be differences in downhole pressures, temperatures and geological formations which call for different types of cement slurries. Through laboratory tests and actual field experience, a desired type of cement slurry, typically defined at least in part by its desired density, is selected for a particular job.
Once the desired type of cement slurry has been selected, it must be accurately produced at the well location. If it is not, adverse consequences can result. During the mixing process, the slurry density has typically been controlled with the amount of water. Insufficient water in the slurry can result in too high density and, for example, insufficient volume of slurry being placed in the hole. Also, the completeness of the mixing process can affect the final properties of the slurry. A poorly mixed slurry can produce an inadequate bond between the casing and the well bore. Still another example of the desirability of correctly mixing a selected cement slurry is that additives, such as fluid loss materials and retarders, when used, need to be distributed evenly throughout the slurry to prevent the slurry from prematurely setting up. This requires there to be sufficient mixing energy in the slurry blending process. More generally it is desirable to obtain a consistent, homogeneous slurry by means of the mixing process. This should be done quickly so that monitored samples of the slurry are representative of the larger volume and so that dry and wet materials are completely or thoroughly combined to obtain the desired slurry.
The foregoing objectives have been known and attempts have been made to try to meet them with continuous mixing systems. In general, these systems initially mix dry cement and water through an inlet mixer which outputs into a tub in which one or more agitators agitates the resulting blend of materials. The process is continuous, with slurry which exceeds the volume of the tub flowing over a weir into an adjacent tub which may also be agitated and from which slurry is pumped down into the well bore. Such systems typically also include some type of recirculation from one or the other of the tubs back into the inlet mixer and the first tub to provide an averaging effect as well as possibly some mixing energy. One or more densimeters are typically used in the systems to monitor density (this is the means the operator uses to determine cement/water ratio), the primary characteristic which is used to determine the nature of the cement slurry.
Despite these mixing systems having significant utility, the oil and gas industry today is seeking systems which provide better mixing than such continuous mixing systems have been able to achieve. It has been observed that in some prior systems the inlet mixer configuration provides inadequate mixing energy and causes, rather than reduces, air entrainment. Excess air entrainment can adversely affect density measurements which in turn affect control systems and thus resultant slurry properties. Inadequate mixing can also allow "dusting" (escape of unmixed dry cement from the mixer). Other shortcomings of at least some prior continuous mixing systems include the necessity of controlling multiple mixing water valves, and in at least one type of system, one of such valves chokes the water source pressure upstream of where mixing occurs so that much of the mixing energy is lost. At least one prior system includes a primary water inlet valve which has an adjustable conical space that can become clogged by debris in the water.
Although the prior continuous mixing systems have served and continue to serve useful purposes, there is the need for an improved mixing apparatus which overcomes one or more, and preferably all, the aforementioned shortcomings. There is the need for a mixing apparatus which has enough mixing energy to mix thick slurries. This would preferably include a high energy primary mixer, more preferably a constant velocity jet type inlet mixing device. This would also include providing increased tank rolling action and increased recirculation rates. There are also the need to reduce air entrainment and the need to increase the available mixing rate at which at least conventional slurries can be mixed. There is also the need to insure better wetting of dry substances which are to be mixed to reduce "dusting." It is also desirable to have a mixing apparatus wherein a single fluid control valve is used for simplifying the control. Such a control valve should be one which is less susceptible to clogging, has a relatively fast response and can be adapted for different gain adjustments. It should also not choke the inlet flow so that significant mixing energy is not lost before reaching the mixing chamber.